PROJECT SUMMARY/ABSTRACT Transcriptional and post-transcriptional mechanisms of gene regulation collaborate to shape the expression of every gene. During early development in all animals, post-transcriptional control of gene expression is uniquely important as no transcription occurs, instead, maternally deposited gene products control the earliest events of embryonic development after egg fertilization. Maternal gene products control development until the maternal- to-zygotic transition (MZT), when maternally deposited mRNAs are degraded and the zygotic genome is activated. In Drosophila melanogaster, nearly 60% of maternally deposited mRNAs are destabilized in a widespread, gene-specific wave of mRNA degradation that is controlled by maternally encoded and zygotically transcribed regulators. Successful handoff from maternal to zygotic control requires this wave of maternal mRNA destabilization, but there is a major gap in our understanding of this fundamental process because the molecular regulators underlying the destabilization are largely unknown. The goal of my proposal is to better understand the post-transcriptional control that operates in the early Drosophila embryo. A major barrier to understanding how maternally deposited mRNAs are destabilized is the fact that nearly two-thirds of maternally deposited mRNAs that are destabilized become transcribed once the zygotic genome is active. This prevents accurate identification of destabilized mRNAs, as the zygotically expressed transcript cannot be distinguished from the maternally deposited form of the same gene. First, I aim to determine the dynamics of maternally deposited and zygotically transcribed mRNAs during the maternal-to-zygotic transition using a method that can accurately distinguish between maternally deposited and zygotically transcribed transcripts. These quantitative measurements will be a valuable resource for the developmental biology and fly communities because they offer a perspective on mRNA dynamics with temporal resolution that is unparalleled by other studies. Second, I aim to identify proteins that regulate the stability of maternally deposited mRNAs. In a preliminary analysis, I annotated the 3? ends of genes expressed during Drosophila embryogenesis and I found that most genes express multiple alternative 3? tandem isoforms. I propose to leverage the naturally occurring differences between the sequence of tandem isoforms to identify RNA sequences that modulate mRNA stability and RBPs that recognize these sequences. These data will fundamentally contribute to our models of mRNA degradation during MZT and through exploring novel regulators of this process I will be in a position to determine why maternal mRNA degradation is essential for embryonic development. Any regulators identified as acting during MZT are also likely to influence mRNA stability in non-developmental contexts, providing information on determinants of mRNA half-life in other cellular contexts.